1. Field of the Invention
The present invention relates to a lens, such as an objective lens which is used in an optical pickup, and, more particularly, to a single lens, a lens system (which is also simply referred to as a xe2x80x9clensxe2x80x9d) comprising a plurality of single lenses and a method of producing the lens.
2. Description of the Related Art
Recently, the numerical apertures of objective lenses of pickups used in recording and reproducing information on and from an information recording medium, such as an optical disk, have become greater so as to cope with the demand for recording and reproducing a vast amount of information.
If the numerical aperture is as large as approximately 0.65, a single aspherical lens in use can be prepared by glass press molding or resin molding or a combination of both. If the numerical aperture exceeds that value, an objective lens cannot be prepared by a single lens, needing a combination of two or more single lenses. In case of combining plural lenses, the centers of the rotational symmetry axes of the lenses should be aligned with one another accurately to thereby make the optical axes coincide with one another. Without the alignment, aberration occurs due to the deviation of the axial center, the image forming performance of the objective lens becomes degraded. In particular, a combination objective lens which has a high numerical aperture exceeding 0.8 requires the positional precision of less than several micrometers.
In a case where two spherical lenses are combined, a scheme called xe2x80x9cbell clamp type centeringxe2x80x9d is widely used to ensure relatively easy processing of the outside diameters with centers being the centers of their spherical surfaces. (See Japan Optoelectro-Mechanics Association, xe2x80x9cOptical Device Processing Technologyxe2x80x9d 84-2-2, p. 5.)
In a case where an aspherical lens is combined with a lens or a lens-barrel, by way of contrast, various problems arise. The aforementioned diameter processing is difficult on an aspherical shape and cannot be carried out with high precision. Further, a sufficient accuracy cannot be obtained when the outside diameter as of the time of pressing is used as it is. With the structure where an aspherical lens is adhered with its outside diameter matched with that of a lens-barrel, therefore, a combination lens including the aspherical lens cannot provide a sufficient precision.
In this respect, xe2x80x9creflection type centeringxe2x80x9d and xe2x80x9ccentering with an auto collimatorxe2x80x9d are used to align, particularly, a combination objective lens having a high numerical aperture that demands a precision of the order of several micrometers. That is, a lens whose outside diameter is smaller than that of a lens-barrel is prepared, centering of the lens is performed while measuring the center of the aspherical surface and the lens is adhered to the lens-barrel when the centering is properly carried out.
According to such a scheme, however, the lens is actually irradiated with light and the aspherical surface is measured while the lens is turned in the lens assembling process. This results in low workability. Such schemes suffer a difficulty in keeping the lens performance at the time of measurement, due to the positional deviation that may occur when the adhesive is cured and the residual stress rises. The same problem arose not only when two lenses were assembled into a single lens system but also when the center of the aspherical surface of a single lens was accurately aligned with the center of a lens-barrel.
Accordingly, the present invention has been achieved in order to solve such problems, and it is an object of the present invention to provide a lens and a production method therefor, which can form a lens accurately and can improve the production yield.
According to the present invention, a lens made of at least one of a resin and glass, includes:
a lens effective diameter area;
a peripheral area formed integral with the lens effective diameter area and located at a periphery of the lens effective diameter area; and
at least one marker area located in the peripheral area and formed on a circle whose center is a center of the lens effective diameter area.
In one aspect of the lens of the present invention, said marker area is a circle whose center is said center of said lens effective diameter area or a recess or a projection which is a part of said circle.
In another aspect of the lens of the present invention, said marker area includes independent small lens shapes.
In a further aspect of the lens of the present invention, said lens is a combination lens acquired by combining a plurality of lenses.
In a still further aspect of the lens of the present invention, said lens has a lens-barrel having a lens seat portion which supports said lens and at which an indication marker area at least partially aligned with a circle whose center is said center of said lens effective diameter area is provided.
According to the present invention, a method of producing a lens having a lens effective diameter area and a peripheral area formed integral with the lens effective diameter area and located at a periphery of the lens effective diameter area, comprises the steps of:
forming, together with the lens effective diameter area and the peripheral area, a lens made of at least either a resin or glass and having at least one marker area located in the peripheral area at the periphery of the lens effective diameter area and formed on a circle whose center is a center of the lens effective diameter area;
preparing a cooperative member having an indication marker area at least partially aligned with a circle whose center is the center of the lens effective diameter area; and
aligning the marker area with the indication marker area and fixing the lens to the cooperative member.
In one aspect of the method of the present invention, said marker area and said indication marker area are circles whose centers are said center of said lens effective diameter area or recesses or projections which are parts of said circles.
In another aspect of the method of the present invention, said marker area and said indication marker area are positioned in contact with each other.
In a further aspect of the method of the present invention, said at least one marker area has independent small lenses which are positioned in such a way that when said center of said lens effective diameter area coincides with a predetermined optical axis, an image of said indication marker area is observed through said small lenses from a predetermined position on said optical axis.
In a still further aspect of the method of the present invention, said cooperative member is a lens or a lens-barrel.